Knowledge from the field of conformity assessment and decision rules

"Is my device OK or not"?

This is the question that users of measuring or test equipment ask themselves after calibration has taken place. The associated answer or statement about compliance with a defined specification is made by evaluating the measurement data recorded during calibration from measurement deviation and measurement uncertainty. This is generally referred to as conformity assessment. With the help of the conformity assessment, the user is certified according to objective criteria whether his measuring instrument is compliant or not.

During calibration, the metrological properties of the calibration item are determined by comparison with precise laboratory standards. These properties are compared in the conformity assessment with a previously established specification limit to determine whether the measuring instrument functions as expected. The specification limit (or also called tolerance) is the limit that indicates the maximum extent to which the value of a measuring device may deviate from the nominal value (reference value).

Measurements do not provide absolutely exact values. In order to assess and further use the result of a measurement, a statement on the quality of the result must be made in addition to the determined estimated value of the measurand. The specification of the measurement uncertainty strengthens the confidence in the measurement results and enables the comparison of different measurements. However, the presence of the measurement uncertainty implies the risk of making an erroneous decision on compliance. The risk for erroneous decisions becomes especially greater if the determined measurement deviation is very close to the specification limit and the measurement uncertainty becomes relatively large compared to the specification.

Decision rules define the way in which measurement uncertainty is to be taken into account in the conformity assessment of calibrations and thus determine the maximum permissible risk of erroneous assessment.

Basically, decision rules for conformity statements are not new for accredited calibration laboratories. Even before the publication of DIN EN ISO/IEC 17025:2018, conformity was issued on the basis of regulations and standards. The choice of the decision rule is up to the test equipment owner.

The choice of the appropriate decision rule is up to the user and is completely independent of whether an ISO or accredited calibration is to be performed. Important in the correct choice of your decision rule is the question of how high the risk of a faulty evaluation may be when calibrating your test equipment.

The following four decision rules are offered at Testo Industrial Services for conformity assessment:

• Confidence level 95: The entire measurement uncertainty is taken into account. The user minimises the risk to only 5% of receiving a calibration item that was incorrectly assessed as conforming.
• Confidence level 85: Only half the measurement uncertainty is taken into account. Thus, cases I and II described in the graph are still within the acceptable range.
• Confidence level 50: The measurement uncertainty is no longer taken into account. All measurement results are accepted where the measurement value is in or on the tolerance limit (cases I to III).
• ILAC-G8: ILAC-G8 is a non-binary conformity assessment and contains four expressions: "pass", "conditional pass", "conditional fail" and "fail". In cases II to IV, the user can decide on the further use of his test equipment.

Conditional fail: At least one measured value is outside the tolerance limit, its measurement uncertainty is still partly within the tolerance limit. The test equipment is marked as "defective" from the point of view of the calibration service provider. In our sample certificate you can see an example of the conformity statement for the case "conditional pass". The conformity statement can be seen on the first page. In addition, the selected decision rule is specified on the last page. 

The decision rules can be selected independently for both factory calibrations (ISO) and accredited calibrations.

Testo Industrial Services offers the decision rule for factory calibrations (VN 50) and for accredited calibrations (ILAC-G8) as standard, unless other decision rules have been specified in the past.